Elevation: Key to Right Design
Contact Lens Practice Pearls
Elevation: Key to Right Design
By John Mark Jackson, OD, MS, FAAO
As the teacher of our contact lens courses, I'm frequently asked for help by former students with challenging contact lens fits. One of the first things I want to see is the patient's topography maps. As we all know, topography can provide a much better view of the corneal shape compared to keratometry, and it can aid considerably in contact lens design. However, practitioners may not be using all the tools that the topographer provides.
Aiding Lens Selection
Axial and tangential maps report corneal curvature, but not the true "shape" of the cornea. When you apply a rigid lens, you want to know where the high and low spots are to see where you will get touch and pooling. Elevation maps provide this information.
The elevation map shows the height of the cornea compared to a "best fit" spherical surface. Warm areas are elevated above the reference surface, and cool areas are below. If you put a lens on the cornea with this curvature, red areas should show touch and blue areas should show pooling. In fact, the reference sphere isn't a bad place to start for base curve selection in many cases.
A practical example of how these maps can help is in selecting a lens to use on an oblate cornea. Figures 1 and 2 show two post-LASIK eyes. If the patient needs a rigid lens, can we use a spherical lens or do we need a reverse-geometry (RG) design? I decide by comparing the elevation in the center of the cornea to about the midpoint of the steep ring that shows on the tangential map—about 3.5mm from center.
Figure 1. The central elevation is about 13 microns, and it averages 15 microns within the ring.
Figure 2. The central elevation is about 30 microns, and within the ring is an average of about 65 microns.
In Figure 1 the central elevation is about 13 microns and averages 15 microns within the ring. That's not much of a difference, so I'm pretty sure a standard GP lens design will work for this cornea and there won't be big areas of pooling and touch.
In Figure 2, however, the central elevation is about 30 microns, and within the ring is an average of about 65 microns. So the central cornea is lower than the ring by about 35 microns. If I used a standard lens and the base curve aligned with the peripheral cornea, there would be excessive pooling and perhaps bubbles in the center of the lens.
In my own clinical experience, when the central clearance exceeds 30 to 40 microns or so, visual acuity may decrease and central bubbles can form. In this case, an RG lens would be a better choice. An RG lens is steeper in the secondary curve to better fit over the steeper midperipheral cornea without excessive lens vault centrally.
Many Different Uses
Oblate corneal fitting is just one example. Elevation maps are useful for just about any type of rigid fit, but especially so for irregular corneas. Make sure you're using all the tools that your topographer offers. CLS
Dr. Jackson is an associate professor at Southern College of Optometry where he works in the Advanced Contact Lens Service, teaches courses in contact lenses, and performs clinical research. You can reach him at firstname.lastname@example.org.
Contact Lens Spectrum, Issue: December 2010